The present invention relates to power conversion circuitry and, more particularly, to an apparatus having an efficient power conversion to accomplish zero voltage switching using a novel switch timing technique.
A known full bridge forward DC-to-DC converter having zero voltage switching typically includes a DC-to-AC converter circuit and an AC-to-DC converter circuit linked together by a high frequency AC link, where isolation is provided on the intermediate AC link. This type of converter is a common circuit topology used to transform electric energy from a source at a given potential to a destination load at a different potential. It typically includes four switches, typically power metal-oxide semiconductor field-effect transistors (MOSFETs), operated in alternating pairs, an input/output isolation and step-up/step-down transformer, an output rectifier, and an output filter. A feedback regulator or controller is included to control the switches.
The main advantages of this converter topology include: constant frequency operation, which allows optimum design of magnetic filter components, pulse width modulation (PWM) control, minimum voltage and current (VA) stresses, and good control range and controllability. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunications and computer systems wherein high voltages are converted down to lower voltages needed to operate the systems. Power converters generally suffer from problems such as switching losses, switching noise and common-mode power transformer noise. Switching losses reduce system efficiency, resulting in greater input power requirements for the same output power. Switching and transformer noise, both conducted and radiated, require filtering to prevent or reduce interference with other sensitive electronic equipment.
When switching devices turn on and off, there is a power loss associated with this action. The power loss relates to the current through the switch and the voltage across the switch during the switching transition. The greatest loss is associated with the turn on of the switch. Zero voltage switching, however, provides a means for eliminating switching losses particularly in higher line voltages. The resulting converter will be more efficient by dissipating less heat. Zero voltage switching is achieved by adding a controlled dead time at the turn on of each stage.
A full bridge converter of this type operates generally as follows. The switches are arranged in two diagonal pairs that are alternately turned on for a fraction of a switching period to apply opposite polarities of the input DC voltage across the primary of the transformer. The operation of the switches produce a zero voltage across the transformer by turning off only one switch of the pair. A switch from the alternate pair is then turned on, allowing the current in the primary circuit to circulate at zero voltage through the two switches. The two switches clamp the voltage across the transformer at zero, thereby eliminating the ringing behavior suffered by the conventional bridge when the switches are off. Thus the switches operate to convert the input DC voltage into an AC voltage required to properly operate the transformer.
Different schemes have been developed to reduce the additional switching losses caused by high frequency switching of conventional converters. For example, semiconductor switching losses can be reduced by using reactive snubber elements. In FIG. 1. a first snubber circuit is implemented in zero voltage switching converter 40. As illustrated, a snubber capacitor 64 may be connected in parallel with a converter semiconductor switch 56, having an anti-parallel connected diode 60. This snubber element 64 tends to limit the rate of rise of voltage experienced by the switching device 56. Thus, snubber element 64 provides an easy method to divert the energy that would be dissipated in the switching device 56 during switching. However, the energy stored in the snubber element 64 needs to be dissipated during a subsequent part of the switching cycle. Each converter semiconductor switch 57-59 are connected in parallel with a snubber capacitor 65-67 and in anti-parallel with a diode 61-63, respectively.
Converters that allow lossless resetting of the reactive snubber energy are referred to as xe2x80x9csoft-switchingxe2x80x9d converters. Soft-switching converters may be broadly categorized as zero voltage switching. Various zero voltage switching schemes and converter topologies have been proposed in an attempt to achieve increased performance over conventional hard-switching converters. Many are disclosed in U.S. Pat. No. 5,781,419 which is incorporated herein.
An exemplary known soft-switching converter circuit topology is the full-bridge PWM converter shown at 40 in FIG. 1. This converter topology 40 achieves PWM control with resonant switching of the converter semiconductor switches. The basic DC-to-DC converter circuit topology 40 includes an input side circuit 42 and an output side circuit 44 with the input circuit 42 and output circuit 44 linked by a transformer 46. The transformer 46 includes a primary winding 48, a secondary winding 50, and is characterized by a leakage inductance 52. The primary 48 of the transformer 46 is connected to a DC input voltage source 54 by a bridge of converter switches that forms the input circuit 42. Four semi-conductor switching devices 56-59, e.g., transistors, form the input side circuit 42 converter bridge. Each switching device 56-59 includes an anti-parallel connected diode 60-63 and parallel connected capacitor 64-67. The output side circuit 44 connects the secondary winding 50 of the transformer 46 to a load, shown here as a resistance load 68, by a diode bridge including four diodes 70-73. An output side filter inductor 74 is connected in series between the diode bridge and the load 68. An output side capacitor 76 is connected in parallel with the load 68. In operation, a PWM controller is used to switch the input side circuit switching devices 56-59 in a sequence to generate an AC signal from the DC voltage source 54 across the primary winding 48 of the transformer 46. The resulting AC signal appearing on the secondary winding 50 of the transformer 46 is rectified by the diodes 70-73 of the output side circuit 44 to provide a DC output voltage to the load 68. The output side inductor 74 and capacitor 76 filter high frequency and transient voltages from the output voltage applied to the load 68. The magnitude of the DC output voltage applied to the load 68 is determined by the magnitude of the DC source voltage, the duty cycle of the PWM controller, and the turns ratio of the transformer 46.
In the DC-to-DC converter topology 40, the leakage 52 and magnetizing inductance""s of the transformer 46 are effectively utilized to achieve zero voltage switching of the switching devices 56-59. The operation of the full-bridge PWM converter 40, to achieve zero voltage switching, is as follows. With input circuit switching devices 56 and 59 initially turned on and conducting, the voltage applied across the primary winding 48 of the transformer 46 will be the voltage level of the voltage source 54, Vin. A corresponding voltage will appear on the secondary winding 50 of the transformer 46, causing an output current to flow through diodes 70 and 73. When switching device 59 in the input side circuit 42 is turned off, the input voltage 54 is disconnected from the primary winding 48. With the input voltage Vin no longer applied to the primary winding 48 of the transformer 46, the current in the output side circuit 44 will free wheel through all of the output side diodes 70-73. A current thus continues to flow through the output side filter inductor 74 to provide power to the load 68. This stored energy in the output side circuit 44 is reflected back through the transformer 46 to the input side circuit 42. The continued flow of current through the primary winding 48 charges the output capacitance 67 of the input side switching device 59, and discharges the output capacitance 66 of input side switching device 58. This causes the anti-parallel connected diode 62 of switching device 58 to conduct. Thus, at this point, switching device 58 can be turned on under zero voltage switching conditions. Since the energy available for achieving zero voltage switching for the leading leg switching devices 58 and 59 is the energy stored in the output filter inductor 74, zero voltage switching can be achieved even at light loads.
After input side switching device 58 is turned on, input side switching device 56 may be turned off, at a later point in time. Current still flows through the primary winding 48 of the transformer 46 due to energy stored in the leakage inductance 52 of the transformer 46. When switching device 56 is turned off, this current charges the output capacitance 64 of switching device 56 and discharges the output capacitance 65 of switching device 57, causing the anti-parallel connected diode 61 of switching device 57 to conduct. Thus, switching device 57 may now be turned on under zero voltage switching conditions, to once again apply the input voltage xe2x88x92Vin to the primary winding 48 of the transformer 46. The switching sequence is then repeated for the turn-off of input side switching devices 58 and 57, and the turn-on of input side switching devices 56 and 59. In order to ensure zero voltage turn-on of the switching devices 56 and 57 in the lagging leg of the input side circuit bridge 42, enough energy needs to be stored in the leakage inductance 52 of the transformer 46 to provide for charging and discharging of the switching device capacitors 64-67 throughout the switching sequence. Since the energy in the leakage inductance 52 is a function of the current to the load 68, zero voltage switching will be lost below a certain load level for the lagging leg switching devices 56 and 57. One way to extend the load range of the DC-to-DC converter 40 is thus by properly sizing the leakage inductance 52 of the transformer 46. In addition the effect of leakage inductance 52 on the circuit is to create a ringing condition with the parasitic capacitance associated with the secondary switches 70-73. The ringing is undamped and results in increased electromagnetic interference. Damping the ringing with a snubber circuit including rectifier diodes only results in additional energy losses.
Conventional soft switching full bridge converters do not enable the primary switch currents to decrease during the reset time of the output choke. In addition, these converters rely upon maintaining the stored energy in primary circuit magnetic elements for driving the switching transitions. The stored energy mechanisms used to drive the switching transitions are typically either relatively small chokes added specifically for the purpose or increased leakage inductance or reduced magnetizing inductance of the main transformer. In either case the amount of energy stored is small.
Thus, there is a need for a simple and efficient power conversion circuit having zero voltage switching.
To address the above-discussed deficiencies of the conventional full bridge converter having zero voltage switching, the present invention is directed to a highly efficient and simple power conversion circuit that accomplishes zero voltage switching (ZVS) using a novel switch timing technique. This circuit eliminates the need for an leakage inductor connected in series with the primary circuit of the converter and rectifier diodes, ultimately reducing size, weight and cost of the converter. A power conversion circuit having features of the present invention includes a full bridge circuit having a plurality of switching devices that intermittently couple the primary winding to the input of the power converter. A transformer couples to the full bridge circuit to receive power from the full bridge circuit into its primary winding. An output side circuit, connected to the secondary winding of the transformer, includes a switching circuit coupled to provide stored magnetic energy to drive the switching transitions of the switching devices in the full bridge circuit. The switching circuit closes at a predetermined time delay after the switching transitions in the full bridge circuit to accomplish zero voltage switching for the plurality of switches in the full bridge circuit. A converter controller provides control signals to the full bridge and the switching circuits.
This power conversion circuit provides a solution using a unique timing mechanism. The switch timing circuit enables the use of the natural stored magnetic energy in the output side circuit of the converter to drive the critical switching transitions to accomplish soft switching for all of the switches in a full bridge forward converter for all transitions.
The previously described version of the present invention has many advantages, including a simple and more reliable energy mechanism for driving the switching transitions and lower switch conduction losses by comparison to conventional full bridge forward converters. Since the energy storage elements used to store the energy to drive the transition are large in the present invention, the stored energy available to drive the transitions is larger than that known in the art. As a result of the large amount of stored energy available to drive the switching transitions, the range over which soft switching can be realized is larger. In addition, the circuit for driving the switching transitions does not require the reversal of the magnetizing current which leads to higher conduction and core losses in known converters. Moreover, the embodiments of the present invention reduce the conduction losses by comparison to other known soft switching full bridge forward converters, since the primary switch currents decrease with the output choke current during the reset time of the output choke. Furthermore, the stored energy used to drive the primary switching transitions is the energy stored in the large output choke(s), which is the natural stored magnetic energy component.